It is frequently desirable to determine the particle size distribution of a particle population having various particle sizes, such as may exist in blood. On the basis of the measured particle distribution, histograms are generated to form a basis for further medical analysis of the blood.
Particle counters, particularly those which generate an electrical signal the amplitude of which is proportional to the volume of particles passing through, or by, a counting zone, are subject to coincidence phenomena during which more than one particle will enter the counting zone almost simultaneously. In such cases, the particle counters will generate a signal proportional to the sum of the volume of the several particles and indicate the apparent presence of a single, larger particle, when in fact two (or more) smaller particles have passed through the counting zone. This creates an undesirable false count.
Typically also, prior art systems for generating particle size histograms have attempted to correct for errors in the observed, or raw counts, by applying correction factors of varying degrees of complexity and based on certain assumptions about the nature of the particle size distribution. Thus, for example, there have been suggested methods for coincidence correction based on an assumption that typical particle populations passing through counters will generate pulses forming a Poisson distribution. Other particle counters have attempted to modify raw counts generated by particle detectors in accordance with the time duration of the generated signals.
Yet other prior art systems vary the generated signal in accordance with the repetition rate of the generated particle signals.
Finally, coincidence error correction has been achieved by applying to the generated raw counts one or more fixed correction factors, based in part on characteristics of the particular particle counter.
Experimental verification of the effectiveness of various coincidence correction schemes has not been entirely successful.
Accordingly, it is a primary object of this invention to provide an improved coincidence error correction method and apparatus possessing improved experimental verifiability.
The occurrence of coincidence is an essentially random, statistical process. As a result, most coincidence error correction systems which employ statistical correction approaches are only approximations and depend on assumptions made about the theoretical particle distribution. This limits the accuracy of the generated data.
It is another object of this invention to provide a coincidence error correction regardless of the type of particle distribution being measured.